The present disclosure relates to methods and devices for fracture mechanics testing. In particular, a drop weight tower is described for initiating a sharp crack in a test material sample. The apparatus has design features intended to increase repeatability and reproducibility of testing conditions.
Fracture mechanics is used to study the initiation and propagation of cracks in materials. The presence of cracks, which may develop over time in a material, can modify the local stresses around cracks, due to stress concentration, such that previous stress analyses of the material are inadequate. Once a crack reaches a critical length, it can propagate through the structure and cause catastrophic material failure. This can occur for many reasons, such as uncertainties in loading or operating environments, material defects, design flaws, construction deficiencies, or failing to perform maintenance. By initiating a crack in a material sample, quantitative relations between the crack length, the material's inherent resistance to crack growth, and the stress at which the crack propagates to cause structural failure can be calculated. Thus, fracture mechanics testing requires initiation of a sharp crack (also sometimes referred as a pre-crack) in the test sample.
Crack initiation in brittle material samples is traditionally achieved by a razor tap method. In this method, a test sample is gripped by a bench vise and a razor blade is held in contact with the test sample. A hammer is then manually dropped on the razor blade to initiate a crack in the test sample. FIG. 1 is a picture showing the shape of the test sample. However, there is limited control over the energy of impact, and therefore the length of crack, which results in poor repeatability and reproducibility of the process. These issues with a conventional razor tap method were previously addressed by 1) minimizing operator error by practicing and improving the operator experience and 2) increasing the number of samples to account for process variability.
It would be desirable to provide a testing apparatus that can provide controlled initiation of a crack in a material sample in a consistent and reproducible manner.